If you look at it objectively, it's pretty clear that consistency with LoL and its lore/visuals is not a high priority at all for LoR.

For example, consider Nocturne's lore in the first place:

Trapped in the spirit realm, and with precious few intruders to sustain it, Nocturne began to starve. The only thing close to the delectable feasts of fear it had once tasted was when mortal minds unknowingly drifted through the ether in the hours of sleep.

Emphasis is mine. The most noteable feature of the Shadow Isles is that there are practically no living, mortal beings there. So according to his current lore it doesn't make sense at all for him to be in the Shadow Isles. There are practically no living beings there to dream and give him a way to manifest into the living world and feed. Which is why in the lore he's not in the Shadow Isles, he's in Demacia.

The reason they put him in Shadow Isles is because their thought process was:

1) We're making Targon.

2) Targon should have Diana.

3) Diana should have a Night/Darkness mechanic

4) We need to find another champion that fits that theme. Nocturne is the best fit, so just stick him in anywhere.

The card mechanics and themes take precedence. There's no way they'd spoil the Nocturne VGU release just for LoR anyways.